1. Field of the Invention
The present invention relates to a driving force transmission control system for transmitting a driving force from one of drive shafts to the other thereof through a transfer clutch, a limited slip differential clutch, and the like which can be optionally connected and disconnected.
2. Description of Related Art
In four-wheel-drive automotive vehicles, the driving force transmitted to front and rear drive shafts and a limited slip differential between front and rear wheels or between right and left wheels are generally controlled by controlling a clutch engaging force of a hydraulic multiple disc clutch and the like.
In the transmission of the driving force as described above, when the clutch engaging force is applied according to a difference between rotational speeds in order to suppress an excessive differential, noise and vibration may be arisen by a stick-slip of the clutch (repetition of a lock state and a slip state of the clutch caused by the difference between a static friction caused when two surfaces of the clutch is stuck to each other and a dynamic friction caused when the two surfaces slip therebetween) if the difference between the rotational speeds is fluctuated by a delay in a control system.
To solve this problem, Japanese Unexamined Utility Model Application Publication No. 5-8097, for example, discloses a technology for mechanically reducing an occurrence of the stick-slip by freely and movably interposing an intermediate member between clutch discs of a clutch constituting a limited slip differential unit.
However, the difference between a coefficient of the static friction and the coefficient of the dynamic friction inevitably exists as long as a friction element typically represented by the clutch is employed, and it is difficult to perfectly prevent the stick-slip by the conventional technology described above.
It is also possible to improve the difference between the coefficient of the static friction and the coefficient of the dynamic friction by a lubricant. However, the lubricant capable of reducing the stick-slip is not always applied in view of other requirements such as the lubrication of gears, and the like.
Further, when the stick-slip is arisen in the clutch and the clutch is in a lock state (in an engagement state of a static frictional coefficient), an indicated control value for engaging the clutch must be largely reduced to permit the clutch to slip again (to shift the clutch to a state in which it is engaged in dynamic frictional coefficient state). In this case, however, the state in which a difference between rotational speeds is smaller than a target value is continued for an abnormally long period of time in a control system. At this time, particularly when the control system has an integral term (past records of control value deviations), the integral term is set to an abnormally small value. Thus, a problem is arisen in that a delay in the control system increases when the clutch begins to actually slip with a result that the stick-slip of the clutch is promoted.
Accordingly, an object of the present invention, which was made in view of the above circumstances, is to provide a driving force transmission control system for an automotive vehicle capable of preventing the occurrence of unpleasant noises and vibrations due to the stick-slip of a clutch element by stably transmitting a torque only in a state of the engagement of the clutch element in the dynamic frictional coefficient state by preventing the clutch element from being locked in the static frictional coefficient state as much as possible even if a delay is caused in the control system.